The horizontal lines etched into the hillside are evidence that a prehistoric
lake once filled the valley. The parallel lines represent the ancient
shorelines of Glacial Lake Missoula. The highest known shorelines are
found at an elevation of 4,200 feet. If the lake existed at this level
today, the top of Mt. Jumbo would be an island and the city of Missoula
would sit beneath 950 feet of water.